Starting control scheme for rectifier-inverter systems

ABSTRACT

A high-voltage electric power converter comprises a plurality of electric valves interconnected in a bridge configuration. In operation the valves are cyclically fired in a predetermined normal sequence by a series of relatively short gate pulses respectively applied thereto. For successful starting, the controls are arranged (1) to idle until the concurrence of a starting command and a proper instant of time for firing any one of said valves, and then (2) to apply simultaneously a gate pulse to that one valve and a gate pulse to the immediately preceding valve in said normal sequence. The controls are preconditioned so that the initial firing angle is within certain limits that ensure continuous conduction by the preceding valve at least from the starting instant to the time at which the succeeding valve is fired.

O United States Patent [151 3,648,147 Leete 1 Mar. 7, 1972 [54] STARTINGCONTROL SCHEME FOR 3,401,327 9/1968 Leppert RECTIFIER-INVERTER SYSTEMS,423,664 H1969 Dewey 3,470,442 9/1969 Ainsworth ..321/2 [72] Inventor:Bernard D. Leete, Newtown Square, Pa.

73 A ee: G neral Elect n Primary Examiner--William M. 8110013,.- 1 e c yAttorney-J. Wesley Haubner, Albert S. Richardson, Jr., [22] Filed; N 0Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [21] Appl.No.: 88,668 [57] ABSTRACT Appliumm Dim A high-voltage electric powerconverter comprises a plurality [63] continuatiomimpan of Sen 754,921,Aug 23 of electric valves interconnected in a bridge configuration. In]968 operation the valves are cyclically fired in a predetermined normalsequence by a series of relatively short gate pulses [52 us. a ..321/2321/2 321/5 respectively aPll"ied sumflfu' "Ming, trols are arranged (1)to idle until the concurrence of a start- [5 l Int. Cl. ..l-l02m 5/40ing command and a proper instant of time for firing any one of [5 ofSearch 4 5 S said valves, and, 13" t0 simultaneously a PUISC n to thatone valve and a gate pulse to the immediately preceding valve in saidnormal sequence. The controls are precondi- [56] References cued tionedso that the initial firing angle is within certain limits that UNITEDSTATES PATENTS ensure continuous conduction by the preceding valve atleast from the starting instant to the time at which the succeedingRe2l,697 1/1941 Lord ..321/45 S valve is fi d 3,315,146 4/1967Paige..... ...321/45 S 3,399,337 8/1968 Stone ..32l/5 27 Claims, 9Drawing Figures Mmm TEfiM/AAl. g 601/7 TERM/M44 Ii I''""' I l I T 4 A 2A A 5/ 39*?" 7 *L i I N 1 I IV 11 i a I L i i i i l I c .r clfirfilki-ilfirii mu i i I i L .1

Patented March 7, 1972 7 Sheets-Sheet 2 INVENTO/F BERNARD fl. LEETE, 5y6499 15. W

ATTOR/VA'Y Patented March 7, 1972 7 Sheets-Sheet 4 NQ M IN VENTOR 65mmR0 0. L 55 r5,

av QQLJ S ATTOAWL'Y Patented March 7, 1972 CURRENT Jf/VSOR 7Sheets-Sheet '7 INVENTOR. BL'RNARD 0 5575,

BY w

A TTORA/[Y STARTING CONTROL SCHEME FORRECTIFIER- INVERTER SYSTEMS Thisapplication is a continuation-in-part of my original US. Pat.application Ser. No. 754,921 filed on Aug. 23, 1968,

This invention generally relates to electric power conver-- sion systemsand more specifically it relates to control means for starting a staticconverter in such a system.

There is a growing interest throughout the world in the bulktransmission of electric power by means of high-voltage direct currenttransmission lines. Opposite ends of such a line are connectedto twoseparate AC electric power systems via a pair of converters. One ofthese converters functions as a rectifier and the other as an inverter,and by controlling which converter operates as arectifier and whichoperates as an inverter, the direction of power flow can be controlledinasmuch as the direction of power flow is from the rectifier terminalover the DC link to the inverter terminal.

A converter comprises the combination of a polyphase power transformerand a plurality of interconnected electric current conducting valves.Each valve can be a mercury arc tube, or a pluralityof semiconductorcells such as thyristors that are operated in unison. Typically thevalves are arranged in a three-phase, double-way bridge configurationhaving three separate AC terminals and positive and negative DCterminals. Each bridge contains six valves, and by sequentially applyingturn-on (gate) pulses to the respective valves in the proper order andin timed relation to the alternating voltages of the three-phaseelectric power system to which theAC terminals of the bridge areconnected, the flow of power between the AC and the DC terminals iscontrolled as desired. To form a l2-pulse converter, the DC terminalsoftwo such bridges are connected in series and the transformer windingsassociated with the AC terminals of the respective bridges arephase-displaced by an electrical angle of 38.

The time at which a valve is fired, measured in electrical degrees fromthe cyclically recurring instant that its anode voltage first becomespositive with respect to cathode, is known as the firing angle. As thefiring angle is increased from zero, the averagemagnitude of therectified voltage between the positive and negative DC terminalsdecreases from its maximum positive level. As the firing angleapproaches 90, the average DC voltage reverses polarity and a converterwill commence to operate as an inverter, whereby power can betransmitted from the DC to the AC terminals.

In the normal steady-state operation of rectifier or inverter bridges,the six valves of a bridge are cyclically fired at intervals of 60electrical degrees in a predetermined sequence. When fired, each willconduct continuously for approximately 120 3 at which time the currentis transferred to asucceeding valve in the sequence. Thus, at least twovalvesare always conducting, and current through-the bridge can becontinuous. In a rectifier-inverter system there need be no particularsteady-state phase.relationship or synchronism between the firing of thevalves in one converter and in the other. At the instant of starting,however, no line current exists to be transferred from one valve toanother, and it is necessary to fire two valves simultaneously.Furthermore, to start a rectifier and inverter system, it may benecessary to simultaneously fire two valves of the rectifier bridge andtwo valves of the inverter bridge, whereby the starting operations ofthe rectifier and inverter must be synchronized.

- Prior art systems have solved the starting problem by applying longgate pulses of 120 duration to the valves. Thus, a firing signal ispresent for the entire conducting period of the valve so that therectifier and inverter can be started at any instant without any .needfor synchronization. It is desirable, however, to use short gate pulsesif possible inasmuch as this minimizes the control power required andimproves the efficiency of the system while lowering its cost.

- withthe accompanying drawings in which:

SUMMARY OF THE INVENTION Itis an object of this invention to provide anelectric power converter starting circuit for a firing system consuminga minimum amount of power.

It is a more specific object of this invention to provide a static powerconverter starting circuit using short duration gate pulses.

It is another object of this invention to provide for smoothtransitionbetween starting and steady-state conditions in aconverterusing short duration gate pulses.

It is another object of this invention to provide a starting controlcircuit for a high-power converter that conducts an initially low valueofcurrent.

Briefly, according to one embodiment of the invention, the convertercontrols include first means for cyclically generating a family of six,1 20 signals for effecting turn-on of the converter valves in apredetermined regular sequence. The controls also include sixvalve-firing means respectively operative when activated to applyrelatively short gate pulses to each of the valves in turn, and normallythese means are respectively activated in response to the existence ofcorresponding control signals generatedby saidfirst means. However, Iprovide blocking means for initially preventing activation of any of thevalve-firing means until disabledby the coincidence of a starting or,recycling command and the leading edge of one of the controlsignals..For coordinating the starting of the local converter with thestarting of another converter connected thereto by a common DC link, theaforesaid disabling action is additionally predicated on thecoincidenceof the leading edge of any one of six similar control signals generatedby the controls associated with the other converter. In either case, toensure successful starting I provide means for preconditioning the localcontrols to establish an initial firing angle within verter).

DETAILED DESCRIPTION The subject matter which is regarded as theinvention is particularly pointed out and distinctly claimed in theconcluding portion of this specification. The invention, both as toorganization and the method of operation, together with further objectsand advantages thereof, may be understood by reference to:the followingdescription taken in connection FIG. 1 is a schematiccircuit diagram ofa portion of an HVDC electric power transmission system;

FIG. 2 is aseries of timecharts showing the interrelationships betweenvarious voltages and control signals;

FIG. 3 is a block diagram'ofa converter control scheme incorporating thestarting controlcircuit of this invention;

.FIG. 4 is a detailed circuit diagram of the terminal starting controlofFIG. 3;

FIG. 5 is a detailed circuit-diagram of the system starting control ofFIG. 3;

FIG. 6, is .a detailed circuit diagram of an alternate embodiment of asystem starting control;

FIG. 7 is a detailed circuit diagram of a second alternative embodimentof asystem starting control;

power system in which a DC link comprising normally positive line 7.andrelatively negative line8 is connected at either end through converters9 and 10 to separate polyphase AC systems at remote terminals (North andSouth). Each converter comprises N controlled valves arranged in atleast one three-phase, double-way bridge configuration having threeACterminals A, B, C which are connected to polyphase conductors (notshown) by means of the set 11 of Y-transformer windings whose voltagesalternate at f hertz in thephase rotation indicated. For purposes ofillustration, f will be assumed equal to 60, and N is shown as 6. Thesix valves, numbered 1 through 6, are fired (turned on) once each cyclein numerical sequence. By controlling the firing angle of the gatingpulses applied to these valves, a converter can be made to operate aseither a rectifier or as an inverter. One converter in a system will beoperated as a rectifier and one converter will be operated as aninverter, and the direction of power flow will be from the rectifierterminal (North) over the DC link to the inverter terminal (South).

In the normal steady-state operation of rectifier or inverter bridges,the six valves of the bridge commence conducting at 60 intervals intheir numbered order and continue to conduct for 120 at which time thevalve current is transferred (commutated) to another valve. Thus currentout of the bridge into a reactive load can be continuous, and in arectifier-inverter system as shown in FIG. 1, there need be noparticular steadystate phase relationship or synchronism between thefiring of the valves at opposite terminals. However, at the instant ofcold starting, no line current exists to be transferred from one valveto another, so it is necessary to turn on two consecutively numberedvalves in the rectifier bridge at the same time. Unless DC line chargingcurrent is expected, it may also be desirable simultaneously to fire twovalves in the inverter bridge in order to establish a continuous paththrough which current can build up. A typical path through valves 1 and2 at each terminal is indicated by the arrows in FIG. 1. If the valveswere gated with short pulses in a normal staggered or sequential manner,there would be no path for current when the first valve is fired, and itwould revert to the blocking state when the gate pulse ceased. Thenthere would likewise be no path for current when the second valve isfired 60 later, and starting could not be accomplished. If the gatepulse for valve 1 persists for 60 or longer, starting can easily occurwhen valve 2 is subsequently fired. The starting method to be describedhereinafter, however, can be used to start a static power conversionsystem when the duration of the gate pulses to the converter valves isshorter than 60 In one embodiment of the present system the gate pulseduration was selected to be 2 /2", and still shorter pulses arecontemplated for future systems.

A requirement for successful start up is that the starting instant occurat a time when there are positive anode voltages on all valvessimultaneously gated. Before starting, the three valves 1, 3, 5 withcommon cathode connection constitute a high-impedance balanced Y-load onthe balanced Y-source 11. Similar conditions exist for the three valves2, 4, 6 with common anode connection. Consequently, the voltage acrosseach valve before starting is essentially the line-to-neutral voltage asis represented in graph A of FIG. 2 for valves 1 and 2. After starting,the common-cathode connection (or plus side 7 of the DC link) is alwaysconnected to one of the three AC terminals through a conducting valve,and again similar conditions exist for the common anode connection.Therefore, a nonconducting valve under operating conditions is subjectedto a line-to-line voltage which is higher in amplitude and 30 lagging inphase compared to the sine waves shown in FIG. 2A. These phase andamplitude differences between valve voltage before and after startingmust be accommodated in order to ensure a successful starting operation.

Another requirement for successful starting is that the current remaincontinuous from one valve firing to the next, which is 60. This may beno problem if the load is sufficiently inductive, but the firing angleshould be limited to less than 60 for purely resistive loads. From graphA in FIG. 2 it can be observed that if starting were attempted bysimultaneously firing valves 1 and 2 at the instant X corresponding to asteady-state delay angle exceeding 60 for valve 2 but 120 for valve 1,the valve 1 current (resistive load assumed) would fall to zero in lessthan 60 thereafter and would thus be extinguished before the firing ofvalve 3, thereby breaking continuity ofthe current path and making itimpossible to start.

For sequentially firing the six valves of a bridge at approximately 60intervals, a corresponding set of control pulses is cyclicallygenerated, and to control the converter the timing of the whole familyof control pulses as a group is shifted relative to the AC line-to-linevoltages. From the sketch of the before-starting valve voltages in graphA it is apparent that any firing angle between 0 and 60 (operatingbasis) for any valve meets the requirement for positive voltage acrossboth that valve and the preceding valve in the sequence and also ensuresthat conduction by the latter will not stop before the next valve isfired. However, it is desirable to retard the firing as much aspossible, consistent with the other requirements, in order to limit theinitial buildup of DC current which may overshoot its normal valuebefore steady-state operation is achieved, and therefore an initialfiring angle a of 50 was selected. While this angle is acceptable forstarting either a 6- pulse converter or a l2-pulse converter, it will berecognized by persons skilled in the art that in order simultaneously tostart all of the bridges of a 24-pulse converter an even shorter delayangle (45 maximum) is dictated by the requirement of positive anodevoltages on both of the initially fired valves in each bridge.

FIG. 3 is a block diagram of a converter control scheme which diagramshows converter starting controls for the North terminal of the FIG. 1system and also shows controls which can be used for coordinatedstarting of the entire system. For clarity, means for controlling onlyone bridge is shown although it is to be understood that similarcontrols will ordinarily be provided for all of the bridges in eachconverter at each terminal of the electric power transmission system. InFIG. 3, signals representative of the actual converter response and thedesired converter response or power order are fed into summing point 13.The output of the summing point is the error between the desiredconverter response and the actual measured response and is fed intoregulator amplifier 14. The DC output signal of amplifier 14 is passedthrough starting bias circuit 15 to a firing time computer 19 which isalso governed by other limits as desired. The computer 19 additionallyreceives signals indicative of the AC system voltages. It responds tothese various inputs by generating, at a frequency that is a multipleoff, a train of discrete pulses which are converted by an appropriatedistributor 20 into a family of six control signals at the respectivepoints 21 through 26 shown in FIG. 3. These control signals arerespectively adapted to effect cyclic firing of the six valves 1 through6 of the local converter in numerical sequence. As is clearly shown inFIG. 2B, the control signals commence at intervals of approximately 60,and each subsists for one-third of a cycle (i.e., which is appreciablylonger than lNf second. The delay angle (a) relating the leading edge ofeach control signal to the positivegoing zero crossing of the anodevoltage of the corresponding valve is normally determined by themagnitude of the error signal that is fed to the firing-time computer19.

The control-signal points 21-26 are shown connected to firing-signallogic means 27 comprising a gate 28 and appropriate logic circuits whichenable additional supervision to be exercised over the timing and theorder of thefiring signals by other permissive and inhibit signals suchas those applied to inputs 29 and 30, respectively, as desired. Theoutputs of the logic circuits are fed to the valve-firing system whichincludes N (six) blocks 31 through 36 representing means respectivelyoperative when activated to transmit relatively short turn-on or gatepulses to each of the corresponding valves 1-6 in turn. The gate pulsesmay be in any suitable form, such as electric current or radiant energy(e.g., light). Activation of each of the blocks 31-36 is achieved inresponse to the existence of the corresponding control signal whenpermitted by the gate 28 and associated logic. 7

The design details of the components 19, 20, 27, and 31-36, shown onlyin block form in FIG. 3, can be conventional and therefore will not beelaborated herein. While the gate 28 has been shown symbolically as aninitially open multipole switch, persons skilled in the art willrecognize that its function can be accomplished in many different ways,such as providing a normally deenergized input bus to AND-logiccircuits, or a normally energized clamp that blocks or suppressesactivation of a bank of transistors. In accordance with the presentinvention, the gate 28 is initially operative to prevent activation ofthe valve-firing system and is subsequently disabled only underconditions ensuring that the unblocking action will occur at aparticular instant relative to the valve voltages and that the firingangle at this instant is of the proper value. Until unblocked, thecontrols idle in which state the timing of the control signals at 21-26is determinable but no gate pulses are actually applied to the valves.For precise control of the unblocking instant, the gate preferablycomprises a normally conducting solid-state switch 28 which inhibits theactivation of any of the means 31-36 in a manner more fully describedbelow. This switch is turned off, thereby disabling the gate andpermitting the valve-firing system to start the converter, at the firstappropriate instant after a start switch 40 is actuated in response toreceipt at 41 of a starting or recycling command. Graph D of FIG. 2illustrates the starting signal produced by the switch 40 when operatedshortly after the control signal for valve 1 commenced, but theunblocking action is not allowed to occur before the leading edge of thenext control signal appears.

To establish the correct instant for unblocking, pulse-mixing means 42is provided for the deriving short pulses or spikes from the leadingedge of the respective control signals which are supplied to it from thedistributor through points 21-26. The train of mixed pulses thusproduced has been illustrated in graph C1 of FIG. 2. Each of the Clpulses indicates that a gate pulse is then due for one of the localvalves 16. It may be desirable in some systems to stretch the mixedpulses into rectangular signals of particular width as shown in graph C2of FIG. 2, and the mixer 42 is additionally designed to serve thispurpose. The formed pulses appear at the output terminal 43 of the mixer42, and they are fed into a coincidence detector 44 along with a similartrain of pulses (shown in graph C3 of FIG. 2 received at terminal 121from the other converter at the remote end of the I-IVDC system. Thecoincidence detector 44 is designed to provide an output only during thetime when pulses from both terminals 43 and 121 are in simultaneousexistence. See graph C4 of FIG. 2. Where it is acceptable to start thelocal converter independently of the remote converter, the detector 44can be omitted or the terminals 43 and 121 Y can be effectivelyinterconnected, and selected pulses in the train generated by thefiring-time computer 19 can be used to energize terminal 43 ifdesiredthereby eliminating the pulse mixer 42.

The output from the coincidence detector 44 is connected to a switchdriver 45 which drives via terminal 174 the blocking switch 28'. Switchdriver 45 can also beused to simultaneously actuate a similar blockingswitch, via terminal 173, in the starting controls for one or moreadditional converters, thereby starting the other converters. As willsoon be described in conjunction with FIG. 5, operation of the switchdriver 45, and hence disabling of the gate28, requires the coincidenceof a starting command 41 (start switch 40 operated) and an output of thecoincidence detector 44, if used (signalling the beginning ofa controlsignal at one of the points 21-26). In the illustration depicted in FIG.2, the start switch was operated after the first control signalcommenced but before the leading edge of the second control signal, andunblocking occurred in response to the latter event. Since each of thesix control signals subsists for appreciably longer than one-sixthcycle, the preceding signal 21 in the normal sequence has not expiredwhen the next signal 22 begins, and the coexistence of two controlsignals is now assured. Therefore at this particular instant, as isclearly shown in graph E of FIG. 2, the logic means 27 will passenergizing pulses to two consecutively numbered'components (e.g.,'31 and32) of the valve-firing system which are immediately activated therebyto transmit short turn-on pulses simultaneously to two valves (e.g., land 2) that are normally fired in succession (see FIG. 2F).

When cold starting an I-IVDC system such as described herein, the DCpower level is initially zero,and assuming the power order is suitablyprogrammed the regulator amplifier 14 will call for the converter outputto increase as rapidly as permitted by built-in limits. For successfuloperation of a bridge under these conditions, I employ bias means 15 forpreconditioning the firing angle computer 19 so as to override otherinputs and to establish an initial firing angle that meets thepreviously explained criteria of ensuring positive anode voltage on eachof the two valves first fired and of avoiding premature extinction ofeither of these valves before current is commutated to the valve that isnext fired in the normal operating sequence.

Preferably the starting bias circuit 15 comprises means for temporarilysuperimposing a DC voltage level on the output of the regulatoramplifier 14. Various arrangements are possible, but as is schematicallyshown in FIG. 3 a bias voltage is taken from a variable resistor 16paralleling a suitable DC source 17 and is applied through a solid-stateswitch 46 across a resistor 18 which is connected between the amplifierl4 and the firingtime computer 19. The variable resistor 16 controls themagnitude of bias voltage, and appropriate .means including the switchdriver 45 is provided to controltheconduction of solidstate switch 46.In response to the local converter being started, a switch-openingsignal is sent by the switch driver 45 to the solid-state switch 46 sothat the starting bias is removed.

The disabling process of the starting bias circuit will be more fullyexplained in conjunction with FIG. 8. Thereafter in a normal manner thefiring angle of the control signals will depend on-the error between thedesired converter output and the actual converter output.

To assist in preadjustingthe timing of the control signals prior tostarting, the prospective firing angle of one of the control signals(e.g., the'first one) can be measured by means ofa firing-angle monitor47 supplied from point 21 and from the AC voltage monitors. The firingangle monitor 47 may be merely an oscilloscope or other suitabledisplay. Then with the starting bias circuit 15 in operation and thecontrols idling, the variable resistor 16 can be adjusted until a firingangle within desired limits is obtained at the local terminal beforeoperating the start switch 40.

FIGS. 2 and 3 show schematically controls that are useful to I start aconverter at one terminal (North) of an HVDC transmission system incooperation with concurrent starting of a remote converter at anotherterminal (South). Persons skilled in the art will recognize that the twopulse trains C2 and C3 respectively applied to the input terminals 43and 121 of the coincidence detector 44 have been shown in FIG. 2 afterappropriate-phase shifts to accommodate for any predictable time delaysin the communication channel that interconnects the converters whichzmaybe separated by long distances. The required coincidence between a pulsein train C2 and'a pulse in train C3 will always be obtained if thefrequencies of oscillation of the firing time computers associated withthe respective converters are not precisely the same. The tendency forthese pulses to beat can be the result of interconnecting two AC systemsof inherently different frequencies f and f or it can be artificiallyimposed, if necessary, by scanning the firing angle of one of theconverters within prescribed limits. As has already been noted, a singleconverter can be independently started by using the same basic schemewith certain simplifying modifications.

FIG. 4 is a detailed circuit diagram of the pulse mixing and formingmeans 42 shown in block form in FIG. 3. In FIG. 4 a DC power supply (notshown) is connected between terminals 56. and 57, and an intermediateterminal 58 is held at zero volts by being connected to ground.

The control signals supplied to the mixer 42 from the respective points21-26 are negative-going potentials which abruptly commence when, as isillustrated in particular for point 21, a normally conducting transistor(59) associated with that point in the distributor 20 is turned off.Each of the six control signals are coupled into the pulse mixer througha separate capacitor 102 and an isolating diode 103 to the base 104 of acommon transistor 105. Two transistors 105 and 106 comprise a monostablemultivibrator for which resistors 109,

110, 111, 112, 113, and 107, diodes 103 and 116, and a zener diode 108form a biasing network. Transistor 105 is normally conducting. However,when the leading edge of any one of the negative control signals arrivesat its base 104 it ceases to conduct and turns off," and a positivepulse appears at its collector 114. Transistor 105 remains off for onlya certain length of time determined by an RC time constant and thenreverts to the conducting state. The RC time constant, which isdetermined by the values of a rheostat 113 and a capacitor 115, is

' preferably selected so that the transistor 105 remains off for nolonger than electrical degrees. Capacitor 115 is charged by conductionof transistor 106 which is turned on when transistor 105 turns off.Current from transistor 106 charges capacitor 115 to a voltagesufficient to turn on transistor 105 once again, which turns offtransistor 106 again.

The above-described operation is repeated every time there is anegative-going control signal at any one of the six inputs 21 26 fromthe distributor 20. Each of these input signals is connected to the base104 of the transistor 105 through a capacitor 102 and a diode 103. Thepulses present at the base 104 of transistor 105 have been illustratedwith inverted polarity in graph C1 of FIG. 2, and the 10 pulses presentat the collector 114 of transistor 105 are shown in graph C2. Theselatter pulses are connected through terminal 43 and conductor 118 tocoincidence detector 44. If desired, a similar monostable multivibratorcircuit (not shown) located at a remote terminal of the HVDCtransmission system provides a second train of pulses C3 at terminal 121which is coupled through conductor 120 to the coincidence detector 44.To prevent spurious signals or noise interference, conductors 118 and120 are shielded and the shields are connected through capacitors 119 toground 58.

As explained before in connection with FIG. 3, coincidence detector 44provides an output C4 only during the time when pulses at its respectiveinput terminals 43 and,121 are in simultaneous existence. The outputfrom the coincidence detector 44 is connected to the switch driver 45which drives the blocking switch 28 when the first coincidence pulse C4appears after the start switch 40 is operated. This will always happenwithin 10 following the leading edge of one of the control signals atpoints 21-26.

In the switch deliver a conductor 122 is normally at ground potential.This conductor leads to the output terminal 174 which is externallyconnected through a resistor 125 to thebase 126 ofa normally conductingtransistor 127 comprising the blockingswitch 28'. As can be seen in FIG.4, a resistor 128 serves to furnish biasing for transistor 127 so as tocause it to be normally turned on in which state it maintains theblocking bus 28 at the potential of the positive power supply terminal56. The firing signal logic means 27 is arranged to prevent thevalve-firing system 31-36 from being activated in response to theexistence of any of the control signals at points 21-26 so long as theblocking bus 28 is clamped to terminal 56. However, this clamp isdisabled and the logic means 27 is unblocked in response to thetransistor 127 being driven off by the application to its base of apositive-going output signal from the switch driver 45.

Switch driver 45 also provides another output signal for starting asecond converter if desired. This leads to the output terminal 173 whichin turn can be connected to blocking switches (not shown) associatedwith the starting controls of other converters. To prevent disturbancesand noise interference, the conductors 122 and 123 are shielded and theshields are connected through capacitors 129 to ground 58.

The output terminal 174 of the switch driver 45 is also coupled througha capacitor to a pulse transformer 51. A secondary winding of the pulsetransformer 51 is connected via a shielded conductor 52 to the startingbias circuit, and a signal induced in this winding serves to release thestarting bias by initiating opening of the solid-state switch 46 shownsymbolically in FIG. 3. To prevent spurious signals and noise, conductor52 has its shielding connected through capacitor 53 to ground.

FIG. 5 shows details of the coincidence detector 44 used in oneparticular embodiment of the invention. The coincidence detector 44comprises a circuit arrangement for perfonning a NAND logic function. Inthe absence of positive pulses at both of the input terminals 43 and 121from the associated pulse forming circuits, the base of a transistor 131is clamped to ground through diodes 132 and 133 so that the outputpotential at collector 134 of this normally off transistor isessentially the same as that of the positive power supply bus 56'.Resistors 135, 136, 137, and 138, and capacitor 139 and diode 140constitute a biasing arrangement for transistor 131. An input signal atonly one of the terminals 43 or121 does not change this situation sincethe base 130 remains grounded through a parallel path. However, whenboth inputs simultaneously exist, both input diodes 132 and 133 are backbiased and the transistor 131 is forward biased into conduction. Thus, anegative-going output pulse is created at collector 134 of transistor131 for the duration of coincidence of the two inputs.

The negative coincidence output pulses atthe collector 134 of transistor131 are coupled through a capacitor 141 and a diode 142 to a transistor143 in the switch driver 45 which as is clearly shown in FIG. 5,preferably comprises a flip-flop circuit. Transistor 143 is immediatelyturned off in response to such a pulse. Ordinarily, turning off of oneside of a flip-flop (transistor) 143 would by reason of the collector144 of transistor 143 being coupled throughresistor 145 and capacitor146 to the base 147 of a companion transistor 148, cause the transistorwhich is the opposite side of the flip-flop to turn on. However, thisflip-flop is unconventional in that transistor 148 is normally preventedfrom turning on by supplying forward current through a base-to-emitterdiode 149 connected so as to negatively bias the base. The magnitude ofthis bias current, as determined by a pair of resistors 150 and 15]connected between the cathode of diode 149 and the negative power supplybus 57', is set so that the transient current from the positive DCsupply bus 56 through resistor 153 at the input side of transistor 148will not exceed the bias current, and therefore transistor 148 remainsoff despite the positive pulses received at its base 147 as aconsequence of the turning off of transistor 143. Transistor 143 returnsto the conducting state in between successive pulses from transistor131.

A start switch shown symbolically at 40 is connected through resistor154 across capacitors 156 and 156. When the start switch 40 is closed bya starting or recycling command, the negative potential at the junctionof resistors 150, 151 is reduced to nearly ground level, therebyremoving the negative bias on base 147 of transistor 148. Removal ofthis bias does not itself cause transistor 148 to turn on if transistor143 is then conducting, since a positive signal is required at base 147to turn transistor 148 on. However, after the negative bias is removed,transistor 148 is turned on at the next instance of transistor 143turning off due to a negative input pulse from the coincidence detector44. Capacitors 155 and 156 provide some degree of filtering acrossswitch 40 in order to prevent premature operation due topositive noisepicked up if switch 40 is remotely located. Resistors 150 and 158provide biasing for transistor 148. Resistor 159 and parallel capacitor160 are connected between the collector 161 of the transistor 148 andthe base 162 of the transistor 143 so that when transistor 148 does turnon a negative bias is imposed on the base 162 of transistor 143 toprevent its return to a conducting state, thus scaling in the flip-flopuntil subsequently reset by opening the switch 40 to terminate thestarting signal. 7

Before starting, transistor 148 is off so that the potential at itscollector 161 is high. Then current is provided through resistor 163 toa DC current amplifier comprising normally conducting transistors 164and 165 and also through resistor 166 to a duplicate DC currentamplifier comprising normally conducting transistors 167 and 168.Resistors 169, 170, 171, and

172 furnish biasing for the. two amplifiers. These .amplifiers producepositivergoing outputs at the respective terminals 173 and 174 when thetransistors 168 and 165 are turned off. In operation, transistors 164,165, 167, and 168 are all turned off when the transistor 148 turns on.The output terminals 173 and. 174 are coupled to.-external circuitsasshown in FIG. .4 and previouslydescribed.

FIG. 6 shows arsimplified alternative embodiment of a portion of thesystem and converter starting controls in which the blocking switchdriver and the starting flip-flop .have .been omitted. In FIG. 6, thepulse-forming and coincidencedetector circuits are substantially thesameasin FIGS.-4 and.5,-and circuitelements identified in FIG. 6 byprimed reference numerals correspond to their unprimed counterpartsinFIGS. 4

and 5. Therefore, reference may be had to the preceding discussion for adescription of .the operation of these circuit elements.

In FIG. 6 the start switch 181 is symbolically illustrated'as a two-polenormally closed switch.-.Before starting and with .the contactsof switch181 closed, a thyristor blocking switch182 is maintained in a blockedcondition (conducting) due to positive current at itsgate 183 regardlessof whether or not the parallel'commutating transistor 184 is turned onas a result of coincidence output pulses received from thecollector 134of transistor 131'. -W hen the start switch 181 is operated (opened) oncommand,.the thyristor control circuit, including resistors 185 and 186and capacitor 187, is .deenergizedand the gate drive. for the thyristorblocking switch 182 is removed, but latchingaction keeps this switchconducting. When, however,1there is a negative-going coincidence outputpulse. from the normally off transistor131 whose collector c134 iscoupled-through resistor 189 to the base-188 of transistor: 184,transistor 184 momentarily turns on. Now the voltage'drop across.transistor' 184 becomes less than the volt- .age drop acrossthyristor'blocking switch 182, and 'the thyristor 182 is commutated off.On the expiration of the coincidence output pulse the transistor 184immediately reverts to its nonconducting state and the potential ofblocking bus'z28 is lowered, thereby disabling its blocking function andenabling the firing signallogic means 27.to energize the valve firingsystem 31-36 in response to the existence of corresponding controlsignals-at points.21-26. Inasmuch as power for the pulse forming circuitcomprising/transistors105 and 106' is, in this embodimennderivedthrougha resistor 190 from .the blocking bus 28, the fall of positivepotential of the blocking bussoreduces the amplitude of pulses createdby athezpulse formingcircuit-that-the transistor 131' in thecoincidencedetector willnow remain continuously off, whereby the starting controlsare sealed in until subsequently reset by closing the start switch 181to trigger the thyristor 182.- Without a sealtin provision, thecoincidence pulses could periodicallyturn on transistor 184 and thereby.upset the normal steady-state operation of the converter. In FIG. 6 thelabeled interconnections with the South terminal are, as previouslyexplained, optional.

FIG. 7 shows still=another embodiment of the system starting control inwhich the startingflip-flop and switch driver of FIG. are replaced by asilicon-controlled switch 191 used to,

drivethrough its anode gate 192 the transistor 127 comprising blockingwitch 28. Circuit elements identified in FIG. 7-by double primes aresimilar to corresponding elements in FIGS. 4 and 5 referred to byunprimed numbers. A start switch 193 is normally closed so thatsilicon-controlled switch 191 isnormally biased on by means of resistors194, 195, and 196 and capacitor 197. Since silicon-controlled switch191is on, current is conductedthrough its anode gate 192 so thattransistor 127 is also conducting. When the start switch 193 is openedon command, there is no immediate change; However, assoon assimultaneous pulses are received from the pulse forming circuits of bothterminals 43 and 121, so that both of the diodes 132" and 133" arereverse-biased, transistor .131" turns on. commutating offsilicon-controlled switch 192. When silicon-controlled switch 192 turnsoff conduction through its. anode gate ceases and transistor 127 isturned off,

uwhichunclamps the blocking -bus28.and allows activation of 199connected through resistors200 and-201, diodes 202 and .203,.andresistorl204 to the base 205 of a transistor 206.

When the silicon-controlled switch 198 is on, conduction occurs throughits anode gate 199 sothat transistor 206 is turned onanamountdeterminedby the adjustment of resistor201. When transistor .206 ison,.a cascaded transistor 207 is also conducting. Transistor 207conducts current from a DC powersupply 17p,,17n through-a diode 209,a-bias polarity .switch..2.1-0,.and the resistor 18. The resultingvoltage drop .across resistor 18 is thestarting'bias voltage. Themagnitude of this voltage, and'hence the firing anglethat'characterizesthe gate pulses:initiallyapplied to the valves to start the converter,depends on the selection and setting of resistor 201 which functionallycorresponds to the :potentiometer16 shown in no.3.

Normally, however, silicon controlled switch 198 is not turned-onanditstanode gate 199 is not conducting. To turn on the switchi1-98 inpreparation for starting, appropriate means shown symbolicallyas.a-normally open switch 211 is momentarily-actuated to connectabiasingnetwork comprising resistors'213 andf2l4 and capacitor'215 across the DCsupply terminals 17p and 17n. The resultant voltage appearing at thecathode gate '212 of silicon-controlled switch 198 turns it on so that,as explainedbefore, the transistors 206 and 207 are initially operativeto establish a starting bias voltage across resistor 18.-It'will now beapparent-that the parts 198, 206 and .207 shown in FIG.'=8 fulfilhthefunction of the solid-state switch'46 previouslydescribed.

A forward-bias is-normallyapplied across the silicon-controlled switch198 by means of resistor 216, diode 217 and resistor 218. Howeverawhenan actuating signal is received from the 'startingbiasrelease'transformer 131 (shown in FIG. 4) over shielded conductor 132, whichconductor is coupled .through a capacitor;219 tothe resistor218, anegative pulse is impressedacrossthe resistor 218 to direct anodecurrent from thesilicon-controlledswitch 198 so as to commutate it off.Thus, current ceasesin the. anode gate 199,- and the transistors206and.207 tend to beturned off; However, while transistors 206 andz207were conducting a capacitor 220 connected between. 17p and-the junctionof :diodes 202 and 203 was charged tossome voltage, andnow dischargeofthis capacitor .throughan adjustable resistor..221 and the.diode 203effectively retards the-decay of forward bias on the transistors 206-and.207. Thusthese transistors will continue to conduct in aterminaltof the power conversion system when the starting bias circuit15 .is. disabled.

- Several adjustable features are provided in the starting bias circuit15. .Bias polarity switch 210, for example, provides for reversingthepolarity of the bias voltage appearing across resistor 18. Rheostat 221provides for adjusting the rate of decay of the starting bias voltagewhen silicon-controlled switch 198 is opened (turned off).

An indicatinglamp 222 is connected between the anode gate 199 ofsilicon-controlled.switch 198 and the positive supply voltage terminal17p via a resistor 224. Zener diode 223 is connected across theindicator lamp 222 to limit the voltage'thereacross. The indicator lampserves to give a visual indication of when silicon-controlled switch 198is on and, hence, when the firing-time computer is preconditioned forstarting.

The invention described above can readily be adapted to control thestarting of a l2-pulse converter comprising two six-pulse bridges whoseDC terminals are connected in series to the DC link and whose ACterminals are respectively connected to a three-phase AC power system bymeans of two sets of polyphase transformer windings which are phasedisplaced 30 electrical degrees with respect to each other. in thiscontext the control components 27, 28, and 31-36 shown in FIG. 3 for onesix-pulse bridge would be duplicated for the additional bridge. So, longas the distributor 20 provides 120 control signals for the firing signallogic of the respective bridges, and the firing time computer 19 ispreconditioned for an initial delay angle no greater than apredetermined amount (e.g., 60), successful starting can be obtained byhaving a common switch driver 45 actuate the blocking switches 28 ofboth bridges simultaneously. In an analogous manner my invention can beused to simultaneously start three bridges in an 18- pulse converter(with a maximum initial firing angle of 50) or four bridges in a24-pulse converter (with a maximum initial firing angle of 45).Alternatively, in cooperation with appropriate bypass switching, therespective bridges of a 12 or more pulse converter can be startedseriatim, each in the 6- pulse mode hereinbefore described.

As was previously mentioned, operation of the switch driver 45 cansupply, via the output terminal 173, the starting command for anotherconverter, and the blocking switch 28, 28 at the other converter can bearranged to immediately respond to this command. If such a scheme isused in a situation where the distal converter is not coordinated withthe local converter by the coincidence detecting function previouslydescribed (i.e., coincidence detector input 121 omitted), and where thealternating voltage of the system to which the AC terminals of thedistal converter are connected is not synchronized with that of thelocal AC system, the initial gate pulses for the valves of the distalconverter will have a random firing angle which might be unpropitious.Improper firing can nevertheless be avoided by initially preconditioningthe firing time computer associated with the distal converter forgenerating gate pulses characterized by a firing angle which is nogreater than the maximum permissible initial firing angle minus 360/Ndegrees, where N is the number of sequentially fired valves forming theconverter. Thus in a l2-pulse converter, for example, the firing anglecould be preset at 20 which equals the maximum angle 60 minus 360/12degrees less a safety margin of On receiving its starting command, theblocking switch of the control means for the l2-pulse converterdiscussed in the preceding paragraph will immediately disable theidle-causing blocking means, whereupon simultaneous gate pulses aregenerated for and applied to the four valves which, in the normal firingsequence, in turn precede the one valve that is due to be fired by thegate pulse next generated, and successful starting of the converter isobtained. This can be illustrated, by way of example, by assuming thatthe incidence of a starting command is 15 electrical degrees before thetime when the control signal for valve 3 in the leading bridge is due tocommence. As soon as this starting command is received, gate pulses forthe two preceding valves 2 and 1 in both bridges of the converter aresimultaneously released. Since the controls were idling with a firingangle of in this example, the actual firing angle for valve 2 in thelagging bridge is 35 which is less than ideal for minimizing currentovershoot but on the safe side for starting. In practice the initialfiring angle can have a value anywhere between 20 and 50, a proper rangefor starting purposes.

It will be observed that the foregoing random mode of operation of myinvention is similar in result to the scanning mode previouslydescribed. In the scanning mode suitable means isprovided initially toprecondition the firing time computer for generating control signalswhose delay angle varies with time between predetermined maximum andminimum values. If the maximum value were set, for example, at 50 andthe minimum at 20 in a l2-pulse converter, the moment of coincidence ofthe starting signal and the leading edge of one of the control signals,and hence the initial firing angle, could actually be anywhere inbetween. Since the initial firing angle is uncertain but within a properrange in either case, it may be preferable to use the random" mode ofoperation at the distal converter of the system, thereby eliminating theneed for detecting pulse coincidence at opposite ends of the DC link.

When the firing angle of the initial gate pulses is less than themaximum permissible value that would ensure successful starting, thepreviously mentioned problem of current overshoot in the DC link may beaggravated. Of course after current builds up to a certain level theregulator amplifier 14 will take over and retard the firing angle asrequired to properly control the steady-state operation of theconverter. But due to timelags in its response, the regulator may acttoo late to correct appreciable initial overshoot. This possibility canbe avoided by modifying the starting bias means 15 so that for a shorttime immediately following operation of the switch driver 45 the firingangle is forced to increase. In other words, the bias means 15 isarranged temporarily to augment its preconditioning effect, therebycausing the firing-time computer to further retard the firing angle ofthe gate pulses, in response to starting the converter. One way toimplement this idea is disclosed in FIG. 8A.

The components and circuits of the modified starting bias means areintended to be the same as shown in FIG. 8 except for those changes andadditions specifically indicated in FIG. 8A. The modified version willoperate in the manner previously described, in so far as its initialpreconditioning effect on the firing time computer is concerned.However, in H6. 8A operation of the switch driver 45 is used to initiatean augmented preconditioning effect instead of immediately disabling thebias means. This can be accomplished by using the signal from the outputterminal 173 of the switch driver to actuate a suitable switch 231 whichwill connect the capacitor 220, via an isolating diode 232 and aresistor 233, to a different voltage setting of the bias-determiningadjustable resistor 201', whereupon the capacitor 220 begins charging,the bias voltage rises, and hence the firing angle progressivelyincreases from the initially established value (e.g., 20) to a higherpreset limit (e.g.,'80).

The above-described firing angle increase (ramp up) occurs over anoperating interval of the order of a few milliseconds, for example,which interval is determined by the time constant of the chargingcircuit for the capacitor 220. Preferably this time constant is selectedso that the rate at which the firing angle increases to its higher limitwill approximate the rate of rise of current in the DC link. Shortlyafter the new, higher limit is attained, the starting bias means isdisabled. Since the increase in firing angle was accompanied by areduction in the DC voltage generated by the converters, the DC linkcurrent is now changing much less rapidly and is closer to the orderedvalue when the regulator assumes control of the firing angle. Thisresults in a more orderly starting process than would be obtained if theregulator were allowed to take over control of the firing angle at atime when a large error exists between the ordered current and theactual current.

The modified starting bias means depicted in FIG. 8A is disabled indelayed response to the converter being started. The disabling action isthe same as that described in connection with FIG. 8 except that it isinitiated by a timer 234 which is coupled through the capacitor 219 tothe resistor 218. The timer itself is activated when the converterstarts, either by a current-sensing circuit 235 which responds to athreshold magnitude of current in the line 7 of the DC link, oralternatively by a signal conveyed over broken line 236 from the outputterminal 173 of the switch driver. The timer 234 can be arranged todelay disabling of the modified bias means for approximately 8milliseconds, for example.

It should be understood that means other than the modified starting biascircuit that is particularly described above can be used toincrementally adjust the timing of the gate pulses during the earlystages of the startingprocess. Forexample, this function canaltematively be incorporated in the companion circuitry that is providedto impose limits on the firingtime computer 19.

While I have shown and described particular embodiments of myinvention,- it will be obvious to those skilled in'theart that variousother changes and modifications maybe. made without departingfrom myinvention in its broader aspects. Forv example, the. same. basicstarting scheme is applicable to either analog or digital forms ofconverter controls. desired, the .distributor20 can bedesigned toprovide control signals'of 180 duration instead of l20?, in which eventan additional but harmless gate pulse will be applied to a third valve.of the bridge at-the instant of starting; Ithas already been pointed outthat the invention is useful for startinga convertereither independentlyor in cooperation with at'least'one otherconverter at the same'or aremoteterminal of the HVDC power transmission system. (Anindividualconverter need not be started simultaneously with a remoteconverter if it is coordinated with appropriate bypass switching, or ifthe initial current that fiows in theDC link is line charging current,or if the remote converter comprises diode bridges.) Coordinatedstarting of two or more converters can be achieved even though theconverters are respectively connected to AC systems of differentfrequencies or they are formed by different numbers of valves.Accordingly, I intend in the. appended claims to cover'all such changesandmodifications as fall within the true spirit'and scope of myinvention.

What I claim as new and desire to secure by Letters'Patent of the UnitedStates is:

.1. For starting an electric power converter comprising a pluralityofelectric current conducting valves connected in a bridge configurationand arranged to commence conducting in a predeterminednormal sequence,thecombination of:

a. control means for cyclically generating a succession of relativelyshort gate pulsesfor the respectivevalves of the converter and forapplying said pulses to corresponding valves-so as'to firethe valves insaidnormalsequence;

b. said controltmeans including meansinitially operative to cause saidcontrol means to idle, in which statethe timing of said gate pulses isdeterminablebut'no gate. pulses are actually applied .to said valves;

c. means for producing a starting signal; and

d. disabling means connected to said'control means and operative todisable said; initially operative means in response to the. coincidenceof said starting signal being produced and one of said gate pulsesbeing-due;

e. said control means being arranged upon operation of said disablingmeans to apply-said one gate pulse tothecorrespondingvalve andsimultaneously to applyagate pulse to the preceding'valve in said normalsequence, thereby starting the converter.

2. For starting an electricpower converter. comprising a plurality ofelectric current conducting valves connected in a bridge configurationand arranged to commence conducting in a predetermined .normal sequence,the combination of:

a. control means for cyclically generating a succession ofrelativelyshort gatepulses for the respective valvesofthe converter andfor'applying said pulses tocorresponding valves so as to fire the valvesin said normal sequence;

b. said control means including meansinitiallyoperative to cause saidcontrol means to idle, i'n which .state the timing of said gate pulsesis determinable but no gate pulses are actually applied'to said valves;

c. additional means initially operative to precondition said controlmeans for generating gate pulses characterized by afiring angle withinpredetermined limits;

d. means -for producing a starting signal; and

e. disabling means connected to said control means and operative inresponse to said starting-signal to disable the initially operative.means which is identified in the above paragraph (b) when one of saidgate pulses is due;

f. said'control means beingsarran'ged upon operation-of said disablingmeans to apply said one gate pulse to the corresponding valve andsimultaneously to apply a gate pulse to the.preceding-valve invsaidnormal sequence, thereby startingthe converter;

3. The combination set forth'in'claim 2 in which said firing angle isnogreater'than 60 electrical degreeson an operating basis.

4. The combination set'forth in .claim 2 in which said additionalmeansincludes means for disabling the same in response to startingtheconverter;

5. The combination setforth in claim 4*in which said additionalmeanssisarranged to progressively change its preconditioning effect inresponse to starting theconverter.

6..Theacombinationrset'forth: in claimw2in' which said additional' meansincludes means fordisabling the same in response. to operation. of. the.disabling means identified in paragraph (c).

7. For startingafirst electric. power converter comprising a pluralityof electric current conducting valves connected in a bridgeconfiguration and. arranged to commence conducting in a; predetermined.normal sequence, which converter is adapted to'beusedin conjunction withanother converter connectedthereto viaa DC link, the combination of:

a. .control means'for' cyclically generating a succession of relativelyshort: gate..pulses' for. the respective valves of saidfirstconverterand for -applying said pulses to corresponding'valves soastofire the valves in said normal sequence;

b.- said.control: meansineluding means initially operative to cause.said control means to idle, in which state the timing of saidgate pulsesis determinable but no gate pulses are actually applied torsaidvalves;meansforproducing astarting signal; and disabling meansnconnected tosaid control means and to said startingzsignalproducing means and alsoadapted to be. connected ito similar control means associated with saidother-converter, saiddisablingmeans being operative todisableasaid:initiallypperativemeans in response to the coincidenceof i.saidstartingsignal,

ii. agate pulse being due'at said first'converter, and

iii; aagatepulse beingdueat said other converter;

e; the. control -means=-forrsaid firstrconverter being arranged uponoperation of said'ldisablingmeans to apply said one gate pulsetothecorresponding valve and simultaneously to 'applya-igateipulse tothe:preceding valve in said normal sequence, thereby startingsaidfirstconverter.

8: For startingsan electric.power-converter in responseto a starting orrecycling'command, said converter comprising N electricvalvesxconnectedzin at.least'one bridge configuration betweenaiDClinkand a polyphase system" whose voltage alternates .atr-atfrequency off hertz, an'improved control circuit comprising:

a; first meansassociated withthe converter for cyclically generatingafamily'of N'control signals for effecting turnonof*the.respective-valves-of'thatconverter in a predeterminedsequence,each" of saidscontrol'signals subsisting appreciablyJonger-than llNf;

b.- avalve-firing system comprising N second means respectivelyoperative when activated to transmit relatively shortturn-on pulses toeach'valvein'tum;

. third means'connectedto said-first means and to said firing systemforrespectively activating each of said second means in response to theexistence of a corresponding control signal; and

d. blockingmeansconnected to said third means for initiallypreventingactivation of any second means untildisabled in response tothecoincidence of i. said command. and

ii. the leading edge of one of said control signals;

9. The improved control circuit of claim 8 in which said first meansisarranged togenerate said control signals in timed relation to thealternating voltage and in which starting bias means is connected tosaid first means for establishing the initial firing angle of saidcontrol signals within predetermined limits.

10. The improved control circuit of claim 9 in which means is providedfor disabling said starting bias means in response to said blockingmeans being disabled.

11. For cold starting an electric power converter comprising 6 electricvalves connected in a double-way bridge configuration between a set ofpolyphase AC conductors and a pair of DC terminals, the combination of:

a. first means for cyclically generating a set of six control signalsfor effecting turn on of the respective valves in a predeterminedsequence, said control signals commencing at intervals of approximatelyone-sixth cycle and each subsisting for appreciably longer thanone-sixth of a cycle;

b. starting bias means for preconditioning said first means so that theinitial firing angle of said control signals can not exceed 60electrical degrees;

c. a valve-firing system comprising six means respectively operativewhen activated to apply relatively short gate pulses to each valve inturn; means interconnecting said first means and said valve-firingsystem for respectively activating each of, said six means in responseto the existence of a corresponding control signal;

c. said interconnecting means including blocking means for initiallypreventing activation of any one of said six means;

means operative while two of said control signals coexist for disablingsaid blocking means and thereby permitting the converter to start; and

g. means responsive to the converter starting for disabling saidstarting bias means.

12. For starting an electric power system comprising at least first andsecond converters respectively connected to opposite ends of a DC link,the first converter comprising N electric valves connected in at leastone bridge configuration between one end of said DC link and polyphaseconductors the voltages across which alternate at a frequency off hertz,and the second converter comprising N electric valves connected in atleast one bridge configuration between the other end of said DC link andpolyphase conductors the voltages across which alternate at a frequencyoff hertz, an improved control circuit comprising:

a. first means associated with each of said converters for cyclicallygenerating a series of control signals for effecting turn on of therespective valves of that converter in a predetermined sequence, thecontrol signals generated by the first means at said first convertersubsisting for appreciably longer than lNf and the control signalsgenerated by the first means at said second converter subsistingappreciably longer than l/Nf'; second means connected to each of saidfirst means and operative in response to the existence of the controlsignals respectively generated thereby for sequentially transmittingrelatively short turn-on pulses to the respective valves of theassociated converter; and d c. third means at each converter forinitially blocking operation of the second means until disabled, saidthird means including means for disabling the same in response to thecoincidence of;

i. the coexistence of at least two control signals generated by thefirst means associated with the first converter and ii. the coexistenceof at least two control signals generated by the first means associatedwith the second converter.

13. For starting an electric power system comprising at least first andsecond converters respectively connected to opposite ends of a DC link,each converter comprising a plurality of electric valves connected in abridge configuration, an improved control circuit comprising:

a. means 19, 20 associated with each of said converters for producing afirst set of control signals; i

b. means 42 for deriving a series of short pulses from the controlsignals of each first set;

c. means 44, 45 for detecting when individual pulses at both convertersare in substantially simultaneous existence; and

d. means 27, 28', 31-36 responsive to the simultaneous existence of saidindividual pulses for deriving from said control signals at eachconverter a set of gate pulses for the valves of the associatedconverter.

14. A cold starting process fora multivalve, double-way electric powerconverting bridge including the steps of a. cyclically generating afamily of staggered, relatively long control signals adapted to activatea valve-firing system for turning on the respective valves of the bridgein a predetermined normal sequence in timed relation to the voltagesacross the AC terminals of the bridge;

b. preadjusting the timing of said control signals to establish aninitial firing angle for said valves within predetermined limits;

c. detecting the leading edges of successive control signals;

d. applying one of said control signals to said valve-firing system atan instant of time determined by the detection of its leading edge,thereby turning on a corresponding valve;

e. simultaneously applying an earlier control signal to saidvalve-firing system to turn on the valve that precedes saidcorresponding valve in said normal sequence; and

f. thereafter applying said control signals in turn to said valve-firingsystem.

15. The process set forth in claim 14 including an additional step ofchanging the effect of said preadjusting step when the bridge starts.

16. The combination set forth in claim 2 in which said additional meansis arranged initially to precondition said control means for generatinggate pulses characterized by a firing angle which varies with timebetween predetermined maximum and minimum values.

17. The combination set forth in claim 2 in which said additional meansis temporarily operative in response to starting the converter to causesaid control means to generate gate pulses characterized by aprogressively increasing firing angle.

18. The combination of claim 7 in which additional means is providedinitially to precondition the control means as sociated with said otherconverter for generating gate pulses characterized by a firing anglewhich varies with time between predetermined maximum and minimum values.

19. The improved control circuit of claim 9 in which said starting biasmeans is arranged temporarily to effect a progressively increasingfiring angle in response to said blocking means being disabled.

20. For starting an electric power converter comprising N electriccurrent conducting valves connected in at least one bridge configurationand arranged to commence conducting in a predetermined normal sequence,the combination of:

a. control means for cyclically generating a succession of relativelyshort gate pulses for the respective valves of the converter and forapplying said pulses to corresponding valves so as to fire the valves insaid normal sequence;

b. said control means including means initially operative to cause saidcon,rol means to idle, in which state the timing of said gate pulses isdeterminable but no gate pulses are actually applied to said valves;

c. additional means initially operative to precondition said controlmeans for generating gate pulses characterized by a firing angle withinpredetermined limits;

d. disabling means connected to said control means and operative inimmediate response to receipt of a starting command to disable theinitially operative means which is identified in the above paragraph(b);

e. said control means being arranged upon operation of said disablingmeans to generate and apply simultaneous gate.

- pulses for and to the M3 valves which, in said normal 17 sequence, inturn precede the one valve that is due to be fired by the gate pulsenext generated after the incidence of said starting command; and

f. means for supplying said starting command to said disabling meanswhen starting is desired.

21. The combination of claim 20 in which said additional means isarranged initially to precondition said control means for generatinggate pulses characterized by a firing angle which is no greater than apredetermined maximum permissible initial firing angle minus 360/Ndegrees.

22. The combination of claim 20 in which said additiona means istemporarily operative in response to starting the converter to causesaid control means to generate gate pulses characterized by aprogressively increasing firing angle.

23. For starting an electric power converter in response to a startingor recycling command, said converter comprising N electric valvesconnected in at least one bridge configuration between a DC link and apolyphase system whose voltage alternates at a frequency off hertz, animproved control circuit comprising:

a. first means associated with the converter for cyclically generating afamily of N control signals for effecting turn on of the respectivevalves of that converter in a predetermined sequence in timed relationto the alternating voltage, each of said control signals subsistingappreciably longer than l/Nf;

b. starting bias means connected to said first means for establishing aninitial firing angle of said control signals no greater than apredetermined maximum permissible initial firing angle minus 360/Ndegrees;

c. a valve-firing system comprising N second means respectivelyoperative when activated to transmit relatively short tum-on pulses toeach valve in turn; third means connected to said first means and tosaid firing system for respectively activating each of said second meansin response to the existence of a corresponding control signal; and

e. blocking means connected to said third means for initially preventingactivation of any second means until disabled in response to saidcommand.

24. For starting an electric power system comprising at least first andsecond converters respectively connected to opposite ends of a DC link,each converter comprising a plurality of electric valves connected in abridge configuration, an improved control circuit comprising:

a. means associated with each of said converters for producing a firstset of control signals;

b. means for deriving a series of short pulses from the control signalsof the first set produced at said first converter;

c. means associated with said first converter and responsive to thecoexistence of a starting command and one of said short pulses forderiving from the control signals at said first converter a set of gatepulses for the valves of that converter, thereby starting said firstconverter;

and initially operative to establish a predetermined firing angle of thecontrol signals of the first set produced at said second converter; and

e. means associated with said second converter and responsive tostarting of said first converter for deriving from the control signalsat said second converter a set of gate pulses for the valves of thelatter converter, thereby starting' said second converter. 25. Theimproved control circuit of claim 24 in which said starting bias meansis initially operative to establish a firing converter.

27. For starting an electric power converter in response to a startingor recycling command, said converter comprising N electric valvesconnected in at least one bridge configuration between a DC link and apolyphase system whose voltage alternates at a frequency off hertz, animproved control circuit comprising:

a. first means associated with the converter for cyclically generating afamily of N control signals for effecting turn on of the respectivevalves of that converter in a predetermined sequence and at apredetermined firing angle, each of said control signals subsistingappreciably longer than l/Nf;

. a valve-firing system comprising N second means respectively operativewhen activated to transmit relatively short turn-on pulses to each valvein turn;

c. third means connected to said first means and to said firing systemfor respectively activating each of said second means in response to theexistence of a corresponding control signal;

d. said third means including blocking means for initially preventingactivation of any second means until disabled;

e. means for disabling said blocking means in response to said command,thereby starting the converter; and

f. bias means connected to said first means for temporarily effecting aprogressively increasing firing angle in response to starting theconverter.

. starting bias means associated with said second converter

1. For starting an electric power converter comprising a plurality ofelectric current conducting valves connected in a bridge configurationand arranged to commence conducting in a predetermined normal sequence,the comBination of: a. control means for cyclically generating asuccession of relatively short gate pulses for the respective valves ofthe converter and for applying said pulses to corresponding valves so asto fire the valves in said normal sequence; b. said control meansincluding means initially operative to cause said control means to idle,in which state the timing of said gate pulses is determinable but nogate pulses are actually applied to said valves; c. means for producinga starting signal; and d. disabling means connected to said controlmeans and operative to disable said initially operative means inresponse to the coincidence of said starting signal being produced andone of said gate pulses being due; e. said control means being arrangedupon operation of said disabling means to apply said one gate pulse tothe corresponding valve and simultaneously to apply a gate pulse to thepreceding valve in said normal sequence, thereby starting the converter.2. For starting an electric power converter comprising a plurality ofelectric current conducting valves connected in a bridge configurationand arranged to commence conducting in a predetermined normal sequence,the combination of: a. control means for cyclically generating asuccession of relatively short gate pulses for the respective valves ofthe converter and for applying said pulses to corresponding valves so asto fire the valves in said normal sequence; b. said control meansincluding means initially operative to cause said control means to idle,in which state the timing of said gate pulses is determinable but nogate pulses are actually applied to said valves; c. additional meansinitially operative to precondition said control means for generatinggate pulses characterized by a firing angle within predetermined limits;d. means for producing a starting signal; and e. disabling meansconnected to said control means and operative in response to saidstarting signal to disable the initially operative means which isidentified in the above paragraph (b) when one of said gate pulses isdue; f. said control means being arranged upon operation of saiddisabling means to apply said one gate pulse to the corresponding valveand simultaneously to apply a gate pulse to the preceding valve in saidnormal sequence, thereby starting the converter.
 3. The combination setforth in claim 2 in which said firing angle is no greater than 60electrical degrees on an operating basis.
 4. The combination set forthin claim 2 in which said additional means includes means for disablingthe same in response to starting the converter.
 5. The combination setforth in claim 4 in which said additional means is arranged toprogressively change its preconditioning effect in response to startingthe converter.
 6. The combination set forth in claim 2 in which saidadditional means includes means for disabling the same in response tooperation of the disabling means identified in paragraph (e).
 7. Forstarting a first electric power converter comprising a plurality ofelectric current conducting valves connected in a bridge configurationand arranged to commence conducting in a predetermined normal sequence,which converter is adapted to be used in conjunction with anotherconverter connected thereto via a DC link, the combination of: a.control means for cyclically generating a succession of relatively shortgate pulses for the respective valves of said first converter and forapplying said pulses to corresponding valves so as to fire the valves insaid normal sequence; b. said control means including means initiallyoperative to cause said control means to idle, in which state the timingof said gate pulses is determinable but no gate pulses are actuallyapplied to said valves; c. means for producing a starting signal; and d.disabling means connected to said control means and to said startingsignal producing means and also adapted to be connected to similarcontrol means associated with said other converter, said disabling meansbeing operative to disable said initially operative means in response tothe coincidence of i. said starting signal, ii. a gate pulse being dueat said first converter, and iii. a gate pulse being due at said otherconverter; e. the control means for said first converter being arrangedupon operation of said disabling means to apply said one gate pulse tothe corresponding valve and simultaneously to apply a gate pulse to thepreceding valve in said normal sequence, thereby starting said firstconverter.
 8. For starting an electric power converter in response to astarting or recycling command, said converter comprising N electricvalves connected in at least one bridge configuration between a DC linkand a polyphase system whose voltage alternates at a frequency of fhertz, an improved control circuit comprising: a. first means associatedwith the converter for cyclically generating a family of N controlsignals for effecting turn on of the respective valves of that converterin a predetermined sequence, each of said control signals subsistingappreciably longer than 1/Nf; b. a valve-firing system comprising Nsecond means respectively operative when activated to transmitrelatively short turn-on pulses to each valve in turn; c. third meansconnected to said first means and to said firing system for respectivelyactivating each of said second means in response to the existence of acorresponding control signal; and d. blocking means connected to saidthird means for initially preventing activation of any second meansuntil disabled in response to the coincidence of i. said command and ii.the leading edge of one of said control signals.
 9. The improved controlcircuit of claim 8 in which said first means is arranged to generatesaid control signals in timed relation to the alternating voltage and inwhich starting bias means is connected to said first means forestablishing the initial firing angle of said control signals withinpredetermined limits.
 10. The improved control circuit of claim 9 inwhich means is provided for disabling said starting bias means inresponse to said blocking means being disabled.
 11. For cold starting anelectric power converter comprising 6 electric valves connected in adouble-way bridge configuration between a set of polyphase AC conductorsand a pair of DC terminals, the combination of: a. first means forcyclically generating a set of six control signals for effecting turn onof the respective valves in a predetermined sequence, said controlsignals commencing at intervals of approximately one-sixth cycle andeach subsisting for appreciably longer than one-sixth of a cycle; b.starting bias means for preconditioning said first means so that theinitial firing angle of said control signals can not exceed 60electrical degrees; c. a valve-firing system comprising six meansrespectively operative when activated to apply relatively short gatepulses to each valve in turn; d. means interconnecting said first meansand said valve-firing system for respectively activating each of saidsix means in response to the existence of a corresponding controlsignal; e. said interconnecting means including blocking means forinitially preventing activation of any one of said six means; f. meansoperative while two of said control signals coexist for disabling saidblocking means and thereby permitting the converter to start; and g.means responsive to the converter starting for disabling said startingbias means.
 12. For starting an electric power system comprising atleast first and second converters respectively connected to oppositeends of a DC link, the first converter comprising N electric valvesconnected in at least one bridge configuration between one end of saidDC link and polyphase conductors the voltages across which aLternate ata frequency of f hertz, and the second converter comprising N'' electricvalves connected in at least one bridge configuration between the otherend of said DC link and polyphase conductors the voltages across whichalternate at a frequency of f'' hertz, an improved control circuitcomprising: a. first means associated with each of said converters forcyclically generating a series of control signals for effecting turn onof the respective valves of that converter in a predetermined sequence,the control signals generated by the first means at said first convertersubsisting for appreciably longer than 1Nf and the control signalsgenerated by the first means at said second converter subsistingappreciably longer than 1/N''f''; b. second means connected to each ofsaid first means and operative in response to the existence of thecontrol signals respectively generated thereby for sequentiallytransmitting relatively short turn-on pulses to the respective valves ofthe associated converter; and c. third means at each converter forinitially blocking operation of the second means until disabled, saidthird means including means for disabling the same in response to thecoincidence of; i. the coexistence of at least two control signalsgenerated by the first means associated with the first converter and ii.the coexistence of at least two control signals generated by the firstmeans associated with the second converter.
 13. For starting an electricpower system comprising at least first and second convertersrespectively connected to opposite ends of a DC link, each convertercomprising a plurality of electric valves connected in a bridgeconfiguration, an improved control circuit comprising: a. means 19, 20associated with each of said converters for producing a first set ofcontrol signals; b. means 42 for deriving a series of short pulses fromthe control signals of each first set; c. means 44, 45 for detectingwhen individual pulses at both converters are in substantiallysimultaneous existence; and d. means 27, 28'', 31-36 responsive to thesimultaneous existence of said individual pulses for deriving from saidcontrol signals at each converter a set of gate pulses for the valves ofthe associated converter.
 14. A cold starting process for a multivalve,double-way electric power converting bridge including the steps of a.cyclically generating a family of staggered, relatively long controlsignals adapted to activate a valve-firing system for turning on therespective valves of the bridge in a predetermined normal sequence intimed relation to the voltages across the AC terminals of the bridge; b.preadjusting the timing of said control signals to establish an initialfiring angle for said valves within predetermined limits; c. detectingthe leading edges of successive control signals; d. applying one of saidcontrol signals to said valve-firing system at an instant of timedetermined by the detection of its leading edge, thereby turning on acorresponding valve; e. simultaneously applying an earlier controlsignal to said valve-firing system to turn on the valve that precedessaid corresponding valve in said normal sequence; and f. thereafterapplying said control signals in turn to said valve-firing system. 15.The process set forth in claim 14 including an additional step ofchanging the effect of said preadjusting step when the bridge starts.16. The combination set forth in claim 2 in which said additional meansis arranged initially to precondition said control means for generatinggate pulses characterized by a firing angle which varies with timebetween predetermined maximum and minimum values.
 17. The combinationset forth in claim 2 in which said additional means is temporarilyoperative in response to starting the converter to cause said controlmeans to generate gate pulses characterized by a progressiVelyincreasing firing angle.
 18. The combination of claim 7 in whichadditional means is provided initially to precondition the control meansassociated with said other converter for generating gate pulsescharacterized by a firing angle which varies with time betweenpredetermined maximum and minimum values.
 19. The improved controlcircuit of claim 9 in which said starting bias means is arrangedtemporarily to effect a progressively increasing firing angle inresponse to said blocking means being disabled.
 20. For starting anelectric power converter comprising N electric current conducting valvesconnected in at least one bridge configuration and arranged to commenceconducting in a predetermined normal sequence, the combination of: a.control means for cyclically generating a succession of relatively shortgate pulses for the respective valves of the converter and for applyingsaid pulses to corresponding valves so as to fire the valves in saidnormal sequence; b. said control means including means initiallyoperative to cause said control means to idle, in which state the timingof said gate pulses is determinable but no gate pulses are actuallyapplied to said valves; c. additional means initially operative toprecondition said control means for generating gate pulses characterizedby a firing angle within predetermined limits; d. disabling meansconnected to said control means and operative in immediate response toreceipt of a starting command to disable the initially operative meanswhich is identified in the above paragraph (b); e. said control meansbeing arranged upon operation of said disabling means to generate andapply simultaneous gate pulses for and to the N/3 valves which, in saidnormal sequence, in turn precede the one valve that is due to be firedby the gate pulse next generated after the incidence of said startingcommand; and f. means for supplying said starting command to saiddisabling means when starting is desired.
 21. The combination of claim20 in which said additional means is arranged initially to preconditionsaid control means for generating gate pulses characterized by a firingangle which is no greater than a predetermined maximum permissibleinitial firing angle minus 360/N degrees.
 22. The combination of claim20 in which said additional means is temporarily operative in responseto starting the converter to cause said control means to generate gatepulses characterized by a progressively increasing firing angle.
 23. Forstarting an electric power converter in response to a starting orrecycling command, said converter comprising N electric valves connectedin at least one bridge configuration between a DC link and a polyphasesystem whose voltage alternates at a frequency of f hertz, an improvedcontrol circuit comprising: a. first means associated with the converterfor cyclically generating a family of N control signals for effectingturn on of the respective valves of that converter in a predeterminedsequence in timed relation to the alternating voltage, each of saidcontrol signals subsisting appreciably longer than 1/Nf; b. startingbias means connected to said first means for establishing an initialfiring angle of said control signals no greater than a predeterminedmaximum permissible initial firing angle minus 360/N degrees; c. avalve-firing system comprising N second means respectively operativewhen activated to transmit relatively short turn-on pulses to each valvein turn; d. third means connected to said first means and to said firingsystem for respectively activating each of said second means in responseto the existence of a corresponding control signal; and e. blockingmeans connected to said third means for initially preventing activationof any second means until disabled in response to said command.
 24. Forstarting an electric power system comprising at least first and secondConverters respectively connected to opposite ends of a DC link, eachconverter comprising a plurality of electric valves connected in abridge configuration, an improved control circuit comprising: a. meansassociated with each of said converters for producing a first set ofcontrol signals; b. means for deriving a series of short pulses from thecontrol signals of the first set produced at said first converter; c.means associated with said first converter and responsive to thecoexistence of a starting command and one of said short pulses forderiving from the control signals at said first converter a set of gatepulses for the valves of that converter, thereby starting said firstconverter; d. starting bias means associated with said second converterand initially operative to establish a predetermined firing angle of thecontrol signals of the first set produced at said second converter; ande. means associated with said second converter and responsive tostarting of said first converter for deriving from the control signalsat said second converter a set of gate pulses for the valves of thelatter converter, thereby starting said second converter.
 25. Theimproved control circuit of claim 24 in which said starting bias meansis initially operative to establish a firing angle which is no greaterthan a predetermined maximum permissible initial firing angle minus theelectrical degrees between successive gate pulses for the valves of saidsecond converter.
 26. The improved control circuit of claim 24 in whichsaid starting bias means is arranged temporarily to effect aprogressively increasing firing angle of the control signals produced atsaid second converter in response to starting of said second converter.27. For starting an electric power converter in response to a startingor recycling command, said converter comprising N electric valvesconnected in at least one bridge configuration between a DC link and apolyphase system whose voltage alternates at a frequency of f hertz, animproved control circuit comprising: a. first means associated with theconverter for cyclically generating a family of N control signals foreffecting turn on of the respective valves of that converter in apredetermined sequence and at a predetermined firing angle, each of saidcontrol signals subsisting appreciably longer than 1/Nf; b. avalve-firing system comprising N second means respectively operativewhen activated to transmit relatively short turn-on pulses to each valvein turn; c. third means connected to said first means and to said firingsystem for respectively activating each of said second means in responseto the existence of a corresponding control signal; d. said third meansincluding blocking means for initially preventing activation of anysecond means until disabled; e. means for disabling said blocking meansin response to said command, thereby starting the converter; and f. biasmeans connected to said first means for temporarily effecting aprogressively increasing firing angle in response to starting theconverter.